Used in industrial measurements technology, especially also in connection with the control and monitoring of automated manufacturing processes, for ascertaining mass flow rates and/or mass flows of media, for example, liquids and/or gases, flowing in a process line, for example, a pipeline, are often measuring systems, which, by means of a measuring transducer of the vibration-type and a measuring—and operating electronics connected thereto and accommodated most often in a separate electronics housing, induce Coriolis forces in the flowing medium and, derived from these forces, generate measured values representing the mass flow, respectively the mass flow rate.
Such measuring systems—most often embodied as in-line measuring devices inserted directly into the course of the process line and, consequently, having a nominal diameter corresponding to a nominal diameter of the pipeline have been known for a long time and have proved themselves in industrial use. Examples of measuring transducers of vibration-type, respectively measuring systems formed therewith, are described e.g. in US A 2003/0084559, US A 2003/0131669, US A 2005/0139015, U.S. Pat. Nos. 4,655,089, 4,801,897, 4,831,885, 5,024,104, 5,129,263, 5,287,754, 5,381,697, 5,531,126, 5,705,754, 5,736,653, 5,804,742, 6,006,609, 6,047,457, 6,082,202, 6,223,605, 6,311,136, 6,360,614, 6,516,674, 6,840,109, 6,851,323, 7,077,014 or Published International Application, WO A 00/02020. Shown therein, in each case, is a measuring transducer having at least one measuring tube, which is point symmetric relative to a symmetry center. The measuring tube has a tube wall of a predetermined wall thickness and composed, for example, of a titanium-, zirconium- and/or tantalum alloy or a stainless steel, and a lumen extending between inlet-side and outlet-side, tube ends of the measuring tube and surrounded by the tube wall. The, for example, S-, respectively Z-shaped, or straight, measuring tube is held oscillatably in an outer support element, for example, of stainless steel, formed, for example, also as a measuring transducer housing jacketing the measuring tube and, consequently, protecting the measuring tube and/or as a support tube at least jacketing the measuring tube. The measuring tube is solidly connected only with its inlet-side tube end with a corresponding inlet-side support end of the outer support element and with its outlet-side tube end with a corresponding outlet-side support end of the outer support element, but is otherwise laterally spaced from the outer support element. The outer support element is, furthermore, adapted, directly, namely via, in each case, a connecting flange provided on each of its two support ends, to be mechanically connected with, in each case, a corresponding line segment of the pipeline, and, thus, to hold the total measuring transducer in the pipeline, respectively to absorb forces introduced from the pipeline. The measuring tube, in turn, opens with each of its two tube ends into the respectively corresponding connecting flanges for the purpose of forming a flow path connecting the two line segments for flow, namely permitting traversing flow from one line segment through the measuring tube to the other line segment.
The at least one measuring tube is, furthermore, adapted, in operation of the measuring system, to guide a flowing medium, for example, a gas and/or a liquid, in its lumen, so as to form with the respective lumen of the connected pipeline the mentioned flow path, and during that to be caused to oscillate about its static resting position for producing as measurable effect Coriolis forces usable for measuring the mass flow rate, and, consequently, the mass flow. Serving as wanted oscillations, namely oscillations of the measuring tube suitable for producing Coriolis forces, are usually oscillations of a natural mode of oscillation inherent to the measuring transducer, the so-called drive, or also wanted, mode, which are excited with an instantaneous resonant frequency of such mode of oscillation, for example, also with practically constant oscillation amplitude. The wanted oscillations generate, as a result of the medium flowing through the measuring tube oscillating in the wanted mode, Coriolis forces, which, in turn, bring about Coriolis oscillations, namely additional oscillatory movements of the measuring tube in the so-called Coriolis mode synchronous with the oscillatory movements of the measuring tube in the wanted mode and, consequently, superimposed thereon. Due to such superimposing of wanted- and Coriolis modes, the oscillations of the vibrating measuring tube registered by means of the sensor arrangements at the inlet side and at the outlet-side have a measurable phase difference dependent also on the mass flow rate.
As already indicated, selected as excitation—or wanted frequency, namely as frequency for the excited wanted oscillations, is usually an instantaneous natural resonant frequency of the oscillation form serving as wanted mode. The wanted mode is, in such case, also so selected that the resonant frequency is, especially, also dependent on the instantaneous density of the medium. As a result of this, the wanted frequency is variable within a wanted frequency interval corresponding to a fluctuation of a density of the medium flowing in the lumen of the measuring tube, whereby by means of market-usual Coriolis mass flow meters besides the mass flow supplementally also the densities of flowing media can be measured. Furthermore, it is also possible, such as, among other things, shown in the above mentioned U.S. Pat. Nos. 6,006,609 or 5,531,126, by means of measuring transducers of the type being discussed directly to measure a viscosity of the flowing medium, for example, based on an excitation power required for exciting, respectively maintaining, the wanted oscillations damped by the medium.
As, among other things, also shown in US A 2005/0139015, US A 2003/0131669, U.S. Pat. Nos. 7,077,014, 6,840,109, 6,516,674, 6,082,202, 6,006,609, 5,531,126, 5,381,697, 5,287,754 or Published International Application WO A 00/02020, measuring transducers of the type being discussed can be formed quite easily by means of only a single measuring tube in such a manner that the particular measuring transducer—, for instance, in contrast to that shown in U.S. Pat. No. 4,655,089—has, except for the mentioned measuring tube, no (other) tube adapted to guide a medium flowing in a lumen and, during that, to be caused to oscillate about a static resting position. As shown in the above mentioned U.S. Pat. No. 7,077,014, in the case of measuring transducers with a single, point symmetric, measuring tube, among other things, also such an oscillatory mode can be actively excited as wanted mode, consequently such oscillations can be selected as wanted oscillations, in the case of which the measuring tube has, in each case, four oscillation nodes, consequently exactly three oscillation antinodes, wherein at least in the case of ideally uniformly formed measuring tube with homogeneous wall thickness and homogeneous cross section, consequently with an equally ideally homogeneous stiffness—and mass distribution—the four oscillation nodes lie in at least one imaginary projection plane of the measuring transducer on an imaginary oscillation axis imaginarily connecting the inlet-side and the outlet-side tube ends with one another, respectively such an imaginary projection plane is inside of the measuring transducer. In the case of measuring transducers with straight measuring tube, the projection plane corresponds practically to a cutting plane cutting the measuring tube imaginarily into two halves, consequently an imaginary bend line representing the wanted oscillations of the measuring tube is coplanar with this projection plane.
Such measuring transducers offered most often with a nominal diameter lying in the range between 0.5 mm and 100 mm and having only a single measuring tube, consequently with only a single tube, through which medium flows, are usually—, for instance, also for preventing, respectively minimizing, undesired, not least of all also transverse, forces dependent on the density of the medium to be measured, respectively, disturbances of the measuring effect associated therewith—supplementally to the aforementioned outer support element, equipped with an additional, inner, support element, which is oscillatably coupled, namely affixed to the measuring tube only with an inlet-side support end and with an outlet-side support end spaced therefrom, consequently is mechanically coupled with its first support end also with the first support end of the outer support element, respectively with its second support end with the second support end of the outer support element. The inner support element is, in such case, furthermore, so embodied and arranged that it is spaced laterally from the measuring tube, as well as also from the outer support element, and that both the inlet-side as well as also the outlet-side support ends of the inner support element are, in each case, spaced from both of the support ends of the outer support element. In the case of such an arrangement of the inner support element on the measuring tube, there extends both between the inlet-side support ends of the two support elements as well as also between the outlet-side support ends of the two support elements, in each case, a free, for example, also straight, tube segment of the measuring tube acting as a spring element between both support elements and allowing movements of the respectively associated support ends of the inner support element relative to the support ends of the outer support element. As a result of this, the measuring transducer enjoys also an oscillatory mode exhibiting most often a resonant frequency different from the wanted frequency and characterized by movement of the entire inner support element relative to the outer support element, in such a manner that also the two support ends of the inner support element are moved relative to the two support ends of the outer support element. In other words, conventional measuring transducers with only a single measuring tube have most often, formed by means of the measuring tube and the inner support element held thereto, an inner part, which is held—, for example, also exclusively—by means of the two free tube segments in the outer support element, and, indeed, in a manner enabling oscillations of the inner part relative to the outer support element. The inner support element composed most often of a steel, for example, a stainless steel or a free-machining steel, is usually embodied as a hollow cylinder at least sectionally enveloping, for example, also coaxial with, the measuring tube or, such as shown, among other things, in the above mentioned U.S. Pat. Nos. 7,077,014 or 5,287,754, for instance, also as a plate, frame or box, and has additionally also a mass, which is most often greater than a mass of the single measuring tube. As shown, among other things, in the above mentioned U.S. Pat. No. 5,531,126, the inner support element can, however, also be formed by means of a blind tube extending parallel to the measuring tube and, in given cases, also essentially equally constructed thereto as regards material and geometry.
For active exciting of the wanted oscillations, measuring transducers of vibration-type have, additionally, at least one electro-mechanical oscillation exciter acting most often centrally on the measuring tube and driven during operation by an electrical driver signal generated by the mentioned driver electronics and correspondingly conditioned to have a signal frequency corresponding to the wanted frequency, e.g. with a controlled electrical current. The oscillation exciter, usually embodied as a type of oscillation coil and, consequently, being of electro-dynamic type, includes most often a first exciter component affixed externally on the measuring tube, namely on a side of the measuring tube not contacted during operation by the medium to be measured, for example, mounted on its tube wall and formed by means of a rod-shaped, permanent magnet, as well as an oppositely placed, second exciter component interacting with the first exciter component. The oscillation exciter serves to convert an electrical power fed by means of the driver signal into a corresponding mechanical power and thereby to generate the exciter forces effecting the wanted oscillations of the measuring tube. In the case of the aforementioned measuring transducers with only a single tube, the second exciter component, which is most often formed by means of a cylindrical coil and, consequently, connected with electrical connecting lines, is usually placed on the inner support element, so that the oscillation exciter acts differentially on support element and measuring tube, so that the inner support element can, during operation, execute oscillations, which are embodied opposite-equally relative to those of the measuring tube, thus with equal frequency and opposite phase.
As, among other things, provided in the above mentioned U.S. Pat. No. 5,531,126, the oscillation exciter in the case of conventional measuring transducers of vibration-type can, however, also be so embodied that it acts differentially on the inner and outer support elements, and, consequently, excites the wanted oscillations of the measuring tube indirectly.
For registering inlet-side and outlet-side oscillations of the measuring tube, not least of all also those with the wanted frequency, measuring transducers of the type being discussed have, furthermore, two oscillation sensors, which are, most often, equally constructed. These work usually according to the same principle of action as the oscillation exciter. Of these oscillation sensors, an inlet-side oscillation sensor is located between the oscillation exciter and the inlet-side tube end of the measuring tube and an outlet-side oscillation sensor between the oscillation exciter and an outlet-side tube end of the measuring tube. Each of the oscillation sensors, for example, electro-dynamic, oscillation sensors, serves to convert oscillatory movements of the measuring tube into an oscillatory signal representing oscillations of the measuring tube. For example, the oscillatory signal can be a measurement voltage dependent on the wanted frequency as well as an amplitude of the oscillatory movements. For such purpose, each of the oscillation sensors includes a first sensor component affixed externally on the measuring tube, for example, connected with its tube wall by material bonding and/or formed by means of a permanent magnet, as well as a second sensor component placed opposite the first sensor component and interacting with the first sensor component. In the case of the aforementioned measuring transducers with only a single tube, the second sensor component, which is most often formed by means of a cylindrical coil and is, consequently, connected with electrical connecting lines, is usually placed on the inner support element, in such a manner that each of the oscillation sensors differentially registers movements of the measuring tube relative to the second support element.
In the case of measuring transducers of the type being discussed, it is, such as, among other things, also mentioned in U.S. Pat. No. 6,047,457 or US-A 2003/0084559, additionally usual to hold the exciter—or sensor component affixed to the measuring tube, in each case, to an extra ring—or annular washer shaped, metal securement element, which is applied on the particular measuring tube and which solidly clamps around the measuring tube, in each case, essentially along one of its imaginary, circularly shaped, peripheral lines. The securement element can be affixed to the measuring tube, for example, by material bonding, for instance by soldering or brazing, and/or by force interlocking, e.g. frictional interlocking, for instance, by pressing externally, by hydraulic pressing or rolling from within the measuring tube or by thermal shrinking, for example, also in such a manner that the securement element is subjected durably to elastic or mixed plastic, elastic deformations and, as a result, is permanently radially prestressed relative to the measuring tube.
A disadvantage of measuring transducers with only a single measuring tube is—besides the frequently very complicated measures for balancing such measuring transducers also over greater density measuring ranges—is to be seen, among other things, in the fact that the measuring transducers react in comparison to measuring transducers with two measuring tubes significantly more sensitively (with movements corrupting the at least one oscillatory signal) to axial clamping forces impressed via a connected pipeline and transferred via the external support element, respectively to shaking forces impressed via the pipeline, and, consequently, have a smaller immunity to such disturbances introduced externally of the measuring transducer, respectively exhibit a comparatively poor mechanical common-mode suppression; this, especially, also for the case, in which the aforementioned shaking forces are in considerable measure directed parallel to the exciter forces driving the wanted oscillations and/or have a force component having the wanted frequency. As a result of this, measuring transducers with only a single measuring tube can, as well as also already discussed in the above mentioned U.S. Pat. No. 5,736,653, in the case of an installation unfavorable in the aforementioned sense, have, at times, increased measuring errors as a result of corrupted oscillation signals.